Henderson-Hasselbalch Calculator

Calculate the pH of a buffer solution using the Henderson-Hasselbalch equation. This tool helps determine the pH of a solution based on the acid dissociation constant (pKa) and the ratio of conjugate base to acid concentrations.

How to Use This Calculator

Enter the pKa value of the weak acid in your buffer system
Input the concentration of the conjugate base in molarity (M)
Input the concentration of the weak acid in molarity (M)
Optionally, adjust the temperature (default is 25°C)
Click Calculate to determine the pH of your buffer solution

Formula Used

pH = pKa + log([A-]/[HA])

Where:

pH = The pH of the buffer solution
pKa = The acid dissociation constant (negative logarithm of Ka)
[A-] = Concentration of the conjugate base
[HA] = Concentration of the weak acid

Example Calculation

Real-World Scenario:

A chemist is preparing an acetic acid buffer solution. They need to determine the pH of a solution containing 0.1 M acetic acid (CH₃COOH) and 0.1 M sodium acetate (CH₃COONa).

Given:

pKa of acetic acid = 4.76
Concentration of acetate ion (conjugate base) = 0.1 M
Concentration of acetic acid = 0.1 M

Calculation:

pH = pKa + log([A-]/[HA])
pH = 4.76 + log(0.1/0.1)
pH = 4.76 + log(1)
pH = 4.76 + 0
pH = 4.76

Result: The pH of the buffer solution is 4.76, which is equal to the pKa of acetic acid because the concentrations of the acid and its conjugate base are equal.

Why This Calculation Matters

Practical Applications

Biological research - maintaining optimal pH for enzymes
Pharmaceutical formulation - drug stability and absorption
Food science - preservation and flavor enhancement
Water treatment - pH control for chemical processes

Key Benefits

Predicts buffer capacity and effectiveness
Helps select appropriate buffer systems for specific pH ranges
Enables precise control of chemical reactions
Essential for maintaining homeostasis in biological systems

Common Mistakes & Tips

pKa is the negative logarithm of Ka (pKa = -log Ka). Make sure you're using the pKa value, not the Ka value, in the Henderson-Hasselbalch equation. If you only have Ka, calculate pKa first by taking the negative logarithm.

The Henderson-Hasselbalch equation technically uses activities rather than concentrations. For dilute solutions, the difference is negligible, but for concentrated solutions (>0.1 M), consider using activity coefficients for more accurate results.

Remember that in the acid-base pair HA ⇌ H+ + A-, HA is the acid and A- is the conjugate base. Make sure you're using the correct values for each. In buffer systems, the conjugate base often comes from a salt (like sodium acetate) rather than the dissociation of the acid itself.

Frequently Asked Questions

A buffer is most effective within ±1 pH unit of its pKa value. For example, an acetic acid buffer with pKa = 4.76 is most effective in the pH range of 3.76 to 5.76. Outside this range, the buffer capacity decreases significantly.

Temperature affects the pKa value of acids, which in turn affects the pH of the buffer. Most acids have higher pKa values at lower temperatures. The temperature coefficient (ΔpKa/ΔT) varies for different acid-base systems, with some being more temperature-sensitive than others.

The Henderson-Hasselbalch equation assumes ideal behavior and uses concentrations instead of activities. It becomes less accurate for: 1) concentrated solutions (>0.1 M), 2) very strong or very weak acids, 3) solutions with high ionic strength, and 4) extreme pH values. In these cases, more complex equations or experimental determination may be necessary.

References & Disclaimer

Scientific Disclaimer

This calculator provides theoretical pH values based on the Henderson-Hasselbalch equation. Actual pH values may vary due to factors such as ionic strength, activity coefficients, temperature effects, and impurities. For critical applications, always verify pH experimentally using calibrated pH meters.

References

Henderson, L. J. (1908). "Concerning the relationship between the strength of acids and their capacity to preserve neutrality." American Journal of Physiology. - Original paper describing the relationship between acid strength and buffer capacity.
Hasselbalch, K. (1910). "Die Berechnung der Wasserstoffzahl des Blutes aus der freien und gebundenen Kohlensäure desselben, und die Sauerstoffbindung des Blutes als Funktion der Wasserstoffzahl." Biochemische Zeitschrift. - The original paper that modified Henderson's equation to its current logarithmic form.
Bates, R. G. (1973). "Determination of pH: Theory and Practice." Analytical Chemistry. - Comprehensive resource on pH measurement theory and practical considerations.

Accuracy Notice

This calculator is most accurate for dilute solutions (<0.1 M) where activity coefficients approach unity. For more concentrated solutions, the calculated pH may differ from experimental values by up to 0.2-0.5 pH units. The calculator also assumes standard temperature conditions unless otherwise specified.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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